The response to inhalation of ambient aerosol particles from indoor and selected outdoor environments will be investigated using high sensitivity elemental analysis by proton induced X-ray emission, PIXE. The response may include both deposition of particles in the respiratory tract and growth in particle size due to exposure to high relative humidity during breathing. Both responses may depend on particle size and on chemical composition of the particles and may be different for different component particles in a mixed aerosol. The different components may be distinguished by their different elemental composition, and the respiratory response of each may be determined in a single experiment. Samples will be taken by 5 stage cascade impactor with fine particle filter and will provide resolution of particles less than 0.25 to greater than 4 mm equivalent aerodynamic diameter. Each size range is analyzed for its elemental consituents by PIXE, including S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Br, Pb, Zr, Cd, and additional heavy metals if present in the aerosol. The nanogram sensitivity of the PIXE technique permits fine particle size resolution at a sampling rate of 1 liter/minute and requires of human subjects only 10 minutes or less of breathing time per sample. Experiments are carried out using ambient aerosol from different actual environments and do not require the use of special tracer materials. A major objective of the research will be to standardize experimental conditions and develop a compact and portable sampling unit which can be used in more specialized medical tests of inhalation response to pollution aerosol particles.